1. Field of the Invention
The present invention relates to an elastic universal joint incorporated into a steering apparatus for an automobile and serving to make a motion of a steering wheel freely transmittable to a steering gear but serving to prevent vibrations of the steering gear from being transferred to the steering wheel.
2. Related Background Art
The automobile steering apparatus is constructed to transmit a motion of a steering shaft rotationally driven by a steering wheel to a steering gear and to give a steering angle to a front wheel. It is common that the steering shaft and an input shaft of the steering gear cannot be arranged on the same straight line. For this reason, a universal joint is provided between the steering shaft and the input shaft, with the result that the motion of the steering wheel is freely transmittable to the steering gear. Further, it has hitherto been practiced that the universal joint was provided with a vibration absorbing ability to prevent a driver from undergoing jarring vibrations transmitted to the steering wheel via the steering gear from the wheels when the automobile would be running. Giving the vibration absorbing ability to the universal joint typically involves the use of a so-called elastic universal joint constructed by incorporating an elastic material such as rubber, etc. into this universal joint so as to prevent the transmission of the vibrations with this elastic material.
Joints conventionally known as this type of elastic universal joints are disclosed in Japanese Patent Laid-Open Application No. 56-39325 (French Patent Laid-Open No. 2464404), Japanese Utility Model Laid-Open Application Nos. 54-82257, 5-83462 and 5-89964 and French Patent Laid-Open No. 2614985. Those conventionally known elastic universal joints have substantially the same structure, and hence a structure of the universal joint disclosed in Japanese Patent Laid-Open Application No. 5-89964 among them will be explained with reference to FIGS. 13 to 15.
This elastic universal joint 1, as illustrated in FIG. 13, includes a shaft 2, a first yoke 4 externally fixed to a front end (a left end in FIGS. 13 and 14) of this shaft 2 through a buffer drum 3, second yoke 5 and a cross shaft 6 for connecting this second yoke 5 to the first yoke 4. A serration shaft 7 is, as depicted in FIGS. 14 and 15, formed at a portion protruding from one end of the buffer drum 3 at the front end of the shaft 2. This serration shaft 7 is serration-engaged with a central bore 9 of a transmission member 8. Accordingly, this transmission member 8 is fixedly provided at the front end of the shaft 2 and rotates together with this shaft 2. Further, protruded segments 10, 10 protruding outward the outer peripheral surface of the buffer drum 3 in the diametrical direction are integrally formed in diametrically opposite positions on the circumference of this transmission member 8.
Among the respective members constituting the elastic universal joint 1, the buffer drum 3 is formed in a cylindrical shape including an elastic material 11 such as rubber, elastomer, etc. That is, this buffer drum 3 is constructed of an internal sleeve 12 and an external sleeve 13 that are each composed of a metal in a cylindrical shape, and these sleeves 12, 13 are disposed concentrically with each other. Then, an outer peripheral surface of the internal sleeve 12 is connected to an inner peripheral surface of the elastic material 11 by baking or bonding, and an inner peripheral surface of the external sleeve 13 is similarly connected to an outer peripheral surface of the elastic material 11. Subsequently, the internal sleeve 12 is fixedly fitted to a front end of the shaft 2, and the external sleeve 13 is internally fixedly fitted to a cylindrical portion 14 of the first yoke 4.
The first yoke 4 includes a pair of first arms 15, 15 extending in an axial direction from diametrically opposite positions of one edge (a left edge in FIGS. 13 and 14) of this cylindrical portion 14 in the axial direction (right-and-left directions in FIGS. 13 and 14). First circular holes 16, 16 are formed coaxially with each other near front edges (left edges in FIGS. 13 and 14) of the first arms 15, 15. Further, notches 17, 17 are formed in the diametrically opposite positions of edge of the cylindrical unit 14 but in areas displaced from the pair of first arms 15, 15. A width dimension W of each of those notches 17, 17 is larger than a width dimension w (W&gt;w) of each of the protruded segments 10, 10 of the transmission member 8. Then, in a state where the shaft 2 is assembled inwardly of the first yoke 4, the protruded segments 10, 10 are received with clearance in the notches 17, 17.
Further, the second yoke 5 includes a pair of second arms 18 spaced away from each other and is fixedly connected to an end of another shaft 19. Second circular holes 20 are formed coaxially with each other near front edges of the second arms 18. Then, four ends of the cross shaft 6 are rotatably supported inwardly of the first and second circular holes 16, 20 through bearings such as radial needle bearings.
The following describes an operation of the thus constructed elastic universal joint 1. When an automobile runs straight, or when a rotational torque applied to the shaft 2 from a steering wheel is small, the protruded segments 10, 10 of the transmission member 8 fixed to the front end of the shaft 2 exist in neutral positions or positions deviating a bit from the neutral positions inwardly of the notches 17, 17 formed in the cylindrical portion 14 of the first yoke 4. In each of those states, the cylindrical portion 14 does not directly contact the transmission member 8 at all. Further, the small rotational torque is transmitted from the shaft 2 via the buffer drum 3 to the first yoke 4. In this case, the elastic material 11 of the buffer drum 3 absorbs the vibrations transferred from the wheels to the first yoke 4 via the steering gear, shaft 19, the second yoke 5 and the cross shaft 6, with the result that the vibrations are not transmitted to the shaft 2.
In contrast with this, when the rotational torque applied to the shaft 2 from the steering wheel is larger as in the case of giving a large steering angle to the front wheel, the respective protruded segments 10, 10 impinge on the inner surfaces of the notches 17, 17. As a result, a large proportion of the rotational torque applied from the steering wheel to the shaft 2 is transmitted via the transmission member 8 to the first yoke 4. In this state, the rotational torque transmitted via the buffer drum 3 is limited. Accordingly, even if the rotational torque transmitted via the elastic universal joint 1 increases, there is eliminated such a possibility that an excessive force acts on the elastic material 11 enough to damage this elastic material 11.
Further, Japanese Patent Laid-Open Application No. 4-42924 discloses an elastic universal joint as shown in FIGS. 23 and 24. The elastic universal joint disclosed in this Publication aims at securing both of a steering stability and a vibration attenuating performance. In this elastic universal joint, circular holes 123, 123 are formed in diametrically opposite positions near a front edge of a housing member 122 fixedly welded to the end portion of a shaft 121. Then, a proximal end portion of the first yoke 104 is internally fixedly fitted in the inside of a buffer drum 103 internally fixedly fitted in the front of the housing member 122. Further, both edges of a pin 124 penetrating the proximal end portion of the first yoke 104 in the diametrical direction are loosely fitted in the circular holes 123, 123.
In the case of the thus constructed elastic universal joint shown in FIGS. 23 and 24, the small rotational torque is transmitted between the shaft 121 and the first yoke 104 via the elastic material 111 constituting the buffer drum 103. Further, when transmitting the large rotational torque, external peripheral surfaces of two ends of the pin 124 impinge on internal circumferences of the circular holes 123, 123, thereby transmitting the torque via this pin 124.
Moreover, Japanese Patent Laid-Open Application No. 6-329033 discloses an elastic universal joint structured as illustrated in FIG. 25. In the elastic universal joint disclosed in this Publication, both ends of the pin 124 fixed to the shaft 121 are loosely engaged with notches 117 formed in a rear edge of the first yoke 104. Further, the buffer drum 103 is interposed between the inner peripheral surface of the housing member 122 including its proximal end fixed to the shaft 121 and the outer peripheral surface of an intermediate portion of the first yoke 104.
In the case of the thus constructed elastic universal joint shown in FIG. 25, the small rotational torque is transmitted between the shaft 121 and the first yoke 104 via the elastic material 111 of the buffer drum 103. Moreover, when transmitting the large rotational torque, the outer peripheral surfaces of the ends of the pin 124 impinge on the inner edges of the notches 117, thereby transmitting the torque via this pin 124.
In the case of the conventional elastic universal joint constructed as shown in FIGS. 13 to 15, the following problems arise in terms of attaining both downsizing and a reduction in weight. To be specific, if an intersecting angle between central axes of a pair of shafts 2, 19 connected to each other via the elastic universal joint 1 is large, with rotations of these two shafts 2, 19, the second yoke 5 largely swings about one shaft of the cross shaft 6. Then, as a result of this swing, the front edge of a second arm 18 of the second yoke 5 approaches the front end surface of the shaft 2 while describing a circular arc locus.
There is almost no case in which the intersecting angle of the elastic universal joint becomes excessive in an ordinary using state. However, providing an intersecting angle that is required in terms of protecting a driver when in a collision. That is, in the event that the front of the automobile is crushed in a collision, the steering wheel should be prevented from being thrust against the driver, and hence it is a common practice that part of a plurality of shafts constituting a steering apparatus is bendable while absorbing an impact energy in the event of a collision. Then, if part of the shaft is bent, the intersecting angle of the elastic universal joint provided at the end of the relevant shaft reaches approximately 90 degrees. Accordingly, securing the large intersecting angle is needed in terms of protecting the driver by making part of the shaft smoothly bendable.
Under such circumstances, the elastic universal joint 1 is required to incorporate such a function that the front edge of a second arm 18 does not interfere with the front end surface of the shaft 2 even in a state where the front edge comes closest to the front end surface. For preventing such an interference, the second yoke 5 and the shaft 2 have hitherto been spaced far away from each other. More specifically:
(1) the first arms 15, 15 of the first yoke 4 are elongated, and the front edge is spaced far away from the front end surface of the shaft 2 by moving the cross shaft 6 leftward in FIGS. 13 and 14, about which the second arm 18 swings; or alternatively,
(2) the cylindrical portion 14 of the first arm 4 is elongated, and the front end surface of the shaft 2 is spaced far away from the second arm 18 by moving the buffer drum 3 and the shaft 2 rightward in FIGS. 13 and 14.
In each of these items (1) and (2), a dimension of the first yoke 4 in the axial direction increases, and this is undesirable because of bringing about rises both in size and weight of the elastic universal joint 1.
In the case of the conventional elastic universal joint, it is difficult to attain steering stability and the vibration attenuating performance and downsizing with a smaller weight. Specifically, in the case of the structure illustrated in FIGS. 13 to 15, the weight thereof can be decreased, but securing both the steering stability and the vibration attenuating performance is difficult. More particularly, an enhancement of the torsional rigidity of the elastic material 11 to secure the steering stability involves increasing the diameter of this elastic material 11 or increasing the hardness of the elastic material 11. In the case of the structure shown in FIGS. 13 to 15, however, the buffer drum 3 including the elastic material 11 is fixedly fitted in the inside of the first yoke 4, and, therefore, the torsional rigidity is hard to enhance with an increased diameter of the elastic material 11. Further, if the elastic material 11 has the increased hardness, it follows that the vibration attenuating performance is deteriorated.
Moreover, in the case of the structure shown in FIGS. 23 and 24, even when the steering stability and the vibration attenuating performance can be secured, it is difficult to reduce the weight. Specifically, in the case of the structure illustrated in FIGS. 23 and 24, the small or large torque is transmitted via the housing member 122, and hence it is required that the rigidity of the housing member 122 be well increased. Accordingly, a plate thickness of the housing member 122 has to sufficiently increase, and a weight of the housing member 122 inevitably rises. Moreover, it is difficult to form the circular holes matching with each other in the housing member 122, the buffer drum 103 and the first yoke 104 (or to match the circular holes previously formed in the respective members 122, 103, 104). As a result, there is a decline in productivity when manufacturing the elastic universal joint.
Further, in the case of the structure illustrated in FIG. 25, it is also difficult to attain steering stability and vibration attenuating performance and decreasing the weight. Specifically, in the case of the structure shown in FIG. 25, the buffer drum 103 and the engaged portions of the two edges of the pin 124 with the notches 117 are so provided as to deviate in the axial direction. Accordingly, if the dimension of the buffer drum 103 in the axial direction is elongated to secure the steering stability and the vibration attenuating performance by increasing a volume of the elastic material 111 of the buffer drum 103, the elastic universal joint increases in size with the result that the weight also rises. Conversely, for downsizing the elastic universal joint and decreasing the weight thereof, the dimension of the buffer drum 103 in the axial direction may be reduced. In this case, the elastic material 111 lacks volume, and it is impossible to secure both the steering stability and the vibration attenuating performance.